It is known that the natural gas combustion reaction produces considerable amounts of steam containing nonnegligible amounts of acids. Thus, the smoke that issues from the boiler and moves toward the stack through an exhaust line is corrosive and laden with humidity. If the temperature of the walls of the exhaust line or stack is lower than the dew point of the smoke, a portion of the steam contained in that smoke will condense on the walls, which is particularly damaging to their mechanical resistance and may bring about a substantial degradation of their structure.
In general the exhausts of a traditional boiler are hotter than the wall of the exhaust line despite the rather considerable cooling they undergo, particularly at the level of the stack. However, condensation may sometimes occur, particularly under unfavorable climatic conditions.
In a condensing boiler, the product of combustion, or "smoke," is cooled in order to recover part of the latent heat of the steam it contains. Therefore the exhausted smoke is saturated with moisture and is all the more liable to condense in the exhaust line or stack, causing the latter to deteriorate.
In order to avoid this problem, the prior art provides a number of solutions.
First, it is possible to protect the exhaust line or stack against the harmful effects of condensation by installing a sealed tube within the exhaust line and connecting said tube to the boiler outlet so that it replaces the old line in exhausting the products of combustion. Of course, such tubes must show good resistance to chemical corrosion by acid condensates. Furthermore, it is necessary to ascertain that the cross-section of the replacement line remains wide enough to exhaust the smoke. If it is no longer wide enough, mechanical extraction methods must be used.
This solution, while effective in some cases, is costly and difficult to implement, particularly where the pipes and lines are very high or very long.
Another solution offered in the prior art consists of lining the inside of the smoke exhaust line with a special mortar that is sufficiently water-repellent to constitute a sort of sleeve for the pipe. However, this solution is difficult to implement, costly, and not always practicable.
Rather than seeking to protect the line, it is also possible to try to dehumidify or dry the "wet" smoke before sending it into the pipe so that it will not condense there. A heat pump evaporator may be made to serve for this purpose. However, for such a solution to be effective in partially drying smoke under the most unfavorable conditions (i.e., when the heat of vaporization is high and the outside temperature low), the heat energy transmitted to the evaporator must constitute a significant share of the power generated by the boiler's burner. In this case, the power output of the heat pump represents a sizable portion of the installed power, which requires significant modifications in the operation of the heat plant, e.g., in the regulating apparatus or in the form of a possible short-circuiting of the recuperator. Furthermore, in order properly to exhaust the smoke that emerges from the heat pump dry but at a relatively low temperature, it is generally necessary to provide for a superheater and/or for mechanical extraction.
This solution is therefore difficult to apply to existing heating plants and, because of the great size of the heat pump and the changes that must be made to the installation, it remains especially costly.
The process of the invention is designed to remove the abovementioned drawbacks, with which the prior art has not dealt satisfactorily.